Electronic circuit boards are conventionally tested by physically contacting test points on the circuit board with electric probes to sense voltages at those test points. The sensed voltages for the circuit board under test are compared to calculated voltages or voltages sensed from a circuit board known to be operating properly to determine whether the circuit board under test is operating properly.
Sophisticated testing apparatus has been developed for automated contact testing of circuit boards. Such automated testing apparatus may have an array of probes arranged in a "bed-of-nails" configuration for simultaneously contacting several different test points on the circuit board under test. In spite of such sophistication, however, the speed of contact testing is limited by the rate at which voltages can be sensed by the contact probes and transmitted to remote test apparatus. The maximum testing speed is often lower than the data rates used in modern high speed circuits, so that contact testing cannot assess the operation of the circuit under test at its normal operating speed. Moreover, space must be provided on the circuit boards for test points, the "bed-of-nails" must be properly configured to align with the test points, and the "bed-of-nails" must make good ohmic contact with each of the test points without damaging the circuit board under test for successful automated contact testing. Consequently high speed, non-contacting test methods and apparatus are desirable.
U.S. Pat. No. 5,006,788 discloses apparatus for automated non-contact testing of electronic circuit boards. The disclosed apparatus comprises a rectangular array of wire loop probes, a tuned receiver, and addressing circuitry for connecting individual probes of the array to the receiver. The apparatus further comprises a signal processor for processing signals detected by the probes, and a display for displaying the received signals as a two-dimensional map. U.S. Pat. No. 5,006,788 was issued on Apr. 9, 1991, in the name of Richard R. Goulette et al and is entitled "Method and Apparatus for Monitoring Electromagnetic Emission Levels".
The apparatus disclosed by U.S. Pat. No. 5,006,788 is operated by placing the array of wire loop probes adjacent to an electronic circuit board and, with the electronic circuit board in operation, successively connecting individual probes of the probe array to the tuned receiver. Local magnetic fields due to electromagnetic emission from the operating circuit board induce currents in the wire loop probes, and these currents are successively sensed by the tuned receiver. The signal processor assembles the successively sampled currents into a data file which is displayed as a two-dimensional map of electromagnetic emissions at the frequency to which the receiver is tuned.
The displayed two-dimensional map of electromagnetic emissions is primarily useful for locating the components on a circuit board which contribute to excessive electromagnetic emissions. Once such components are located, they can be redesigned or replaced in an effort to reduce electromagnetic emissions for compliance with government standards (e.g. electromagnetic emission standards set by the Federal Communications Commission in the U.S.A).
U.S. Pat. No. 5,006,788 suggests that the disclosed apparatus can also be used to distinguish properly operating circuit boards from improperly operating circuit boards. The improperly operating circuit boards should have a different pattern of current flow leading to a different pattern of electromagnetic emissions as compared to properly operating circuit boards. Assuming that all properly operating circuit boards have a sufficiently similar pattern of electromagnetic emissions, the improperly operating circuit boards should be distinguishable as those circuit boards which do not have the pattern of electromagnetic emissions which is characteristic of properly operating circuit boards.
Unfortunately, as measured by the apparatus disclosed in U.S. Pat. No. 5,006,788, the electromagnetic emissions of properly operating circuit boards do not always have a pattern of electromagnetic emissions which is distinct enough to permit reliable discrimination of properly and improperly operating circuit boards. In fact, the difference in electromagnetic emission maps at a specific frequency for two properly operating circuit boards (as measured by the apparatus disclosed in U.S. Pat. No. 5,006,788) is sometimes greater than the difference in emission maps for properly and improperly operating circuit boards so that improperly operating circuit boards cannot be readily distinguished from properly operating circuit boards in some cases.